Cow milk is an excellent food containing almost all nutrients and well absorbed by digestion and is especially suitable as a diet for the sick. Weight ratio of phosphorus to calcium in cow milk (0.9:1), however, is undesirable from the viewpoint of calcium absorption, because approximately 42% of the calcium in the milk exists in colloid phase such as phosphate salts which is in difficultly absorbable state. It is desirable from the viewpoint of calcium absorption that weight ratio of phosphorus to calcium in food is around 1:2 as it is in human milk. Higher levels of phosphorus than the above will form calcium phosphate to insolubilize the calcium and hinder absorption of the calcium. Whereas phosphorus excess in such a degree as mentioned above does not cause troubles in healthy persons, it is desired to decrease phosphorus content in cow milk to a phosphorus to calcium ratio of about 1:2 when taken by kidney-disease patients with deteriorated calcium-absorbing function, persons of old age, patients with calcium-metabolism abnormality, and hyperphosphatemia patients. For this purpose, phosphorus in cow milk which is contained in an amount of approximately 3 g per 100 g of protein should be reduced to 1.0-2.0 g.
Heretofore, for reducing phosphorus content in cow milk there are known a process according to which an alkali solution of casein separated from cow milk is heattreated at 60.degree.-150.degree. C. for 2 sec. to 60 min. and the treated casein solution is subjected to an ion-exchange treatment (Japanese Patent Publication No. 25697/1971) and a process according to which skimmed milk is acidified to a pH of 5.2 -6.0 and the acidified skimmed milk is passed through an ion-exchange material (Japanese Patent Laid-open to Public No. 256342/1985).
According to the above-mentioned prior-art methods, the treatment with an ion-exchange material is associated with a disadvantage that other useful acid and mineral components are also removed together with phosphorus, although the phosphorus content in cow milk can be reduced. Process by means of a membrane dialysis instead of an ion-exchange material which is also known is disadvantageous in that nutrients with a molecular weight similar to that of phosphorus are simultaneously removed.
Therefore, the object of the invention is to provide phosphorus-reduced cow milk with the content of only phosphorus in cow milk reduced to the degree that is contained in human milk and a process for preparing the same.